Associating multiple target groups with your Fargate service

Associating multiple target groups with your Fargate service

One small problem with the Fargate configuration created for our gRPC/REST example code is the fact I had to manage the gRPC Target Group manually. I.e., every time I had to update the service (to deploy a new image, for example), my container would be re-deployed to a new instance with a new IP address. The REST Target Group would be updated automatically by the system, but since my service wasn’t aware of the gRPC Target Group, I had to manually register the new IP address there.

I thought to myself: “there must be a better way to do this”. I poked around the ECS Console UI, thinking I probably missed the option to register multiple Target Groups with my service, without luck. Howerver, a simple Google search pointed me to this gem.

So, you can do it, but you have to use the CLI. Which is great for me: I usually prefer the command line and was looking for an excuse to learn how to manage my service using aws CLI. FWIW, I’m using aws-cli/1.15.58.

It was surprisingly easy! I got it working in 20 minutes flat. I knew I would have to create a new service, since you can’t update a Fargate service Load Balancer-related configurations after service creation. So my approach was basically dumping the existing service (created via the Console UI), modifying the loadBalancers config entry, and creating a new service based on that.

Dump the existing service

aws ecs describe-services --services <service name> --cluster <cluster name>

Dump all Target Groups to retrieve IDs

aws elbv2 describe-target-groups

Generate a skeleton json file for your new service

aws ecs create-service --generate-cli-skeleton > my-service-config.json

Create new service

aws ecs create-service --cli-input-json file://my-service-config.json

After generating the skeleton json file, I populated it with the relevant entries from the json dump of the existing service. I then updated loadBalancers entry and picked a new cool name.

When attempting service creation, I had to fix all of the little problems in my json config file, but the CLI output made that pretty easy.

The one error that took a bit more investigation was You cannot specify an IAM role for services that require a service linked role. Luckily, as it’s usually the case when working with AWS, there was a good stackoverflow thread and doc to help me out. The solution was to simply omit the "role" entry in the json config, to let the system use the “service-linked” role.

My next step now will be deleting the old service, since I won’t need that any longer. Overall this was a lot smoother than I had anticipated.